You will need to have 1 million credits for each character you wish to unlock this companion on and legacy level 40 to purchase a mercenary contract that will unlock this new companion. Go to Cartel Bazaar on the south end of the fleet (north end for Republic) once Patch 2.3 arrives on live servers. You will see a droid that sells you a mercenary contract for 1 mil credits (requires legacy level 40) and using it will bring up a short conversation where you are introduced to Treek.